This invention relates to the field of heating units, especially heating units intended for use in processes requiring precisely-controlled amounts of heat.
The present invention is an improvement over the circuits described in U.S. Pat. No. 6,100,510, the disclosure of which is incorporated by reference herein. The cited patent discloses a bridge circuit, in which a heating element comprises one arm of the bridge. The present invention can be used in essentially the same environment, and for the same purpose, as the circuit shown in the cited patent.
An important aspect of the invention described and claimed in U.S. Pat. No. 6,100,510 is the limitation of the sensing period duty cycle, thereby reducing unwanted heat generated by the sensing current. The circuit described in the above-cited patent samples heater resistance every 16.6 milliseconds, and is commercially useful with heater element materials having positive temperature coefficient (PTC) values as low as about 500 PPM.
The present invention has resulted in significant improvements to the original circuit described in the above-cited patent. These improvements further reduce the sensing circuit duty cycle, which reduces dissipated heat even further than in the original circuit. In turn, a short duty cycle allows the use of very high peak sensing currents, which permit the circuit to operate with heating element materials having PTC values as low as 50 PPM.
In another embodiment of the present invention, quad comparator circuitry has been devised that provides sampling rates of either 16.6 ms or 8.3 ms, and also allows the sensing period to be more precisely tailored.
In still another embodiment, switching circuitry is provided whose performance approaches that of the quad comparator circuitry, while using fewer components than are required by the quad comparator. In particular, the circuit of the present invention provides sensing pulses having an amplitude which is relatively unaffected by changes in line voltage.